1. Field of the Invention
The present invention relates to a glass substrate for a magnetic disk that is used in a magnetic disk drive, such as a hard disk drive.
2. Description of the Related Art
A torus-shape glass substrate having a circular hole at the center of a circular plate has been used as a glass substrate for a magnetic disk that is used in a magnetic disk drive, such as a hard disk drive (see Japanese Patent Application Laid-open No. H6-198530). This glass substrate is produced in the following manner, for example. Firstly, thin glass plates are cored and shaped into a plurality of torus-shape glass substrates. Then, these glass substrates are polished at the same time by a batch-type double-sided polishing machine so that values defining patterns of their main surfaces is set, for example, sufficient for high-density recording (see Japanese Patent Application Laid-open No. H10-241144 and Japanese Patent Application Laid-open No. 2000-348330). For example, in Japanese Patent Application Laid-open No. H10-241144, the pattern of the main surface of the glass substrate is defined by surface roughness, more particularly, it is defined that a maximum height Rmax≦15 nm, an arithmetic-average roughness Ra≦1 nm, and a root-mean-square roughness Rq≦1.5 nm. In contrast, in Japanese Patent Application Laid-open No. 2000-348330, the pattern of the main surface is defined by average height of waviness, more particularly, it is defined that an average height of micro-waviness at periods from 2 μm to 4 mm is equal to or lower than 1.27 nm, and an average height of waviness at periods from 300 μm to 5 mm is equal to or lower than 1.0 nm.
To meet requests for higher-capacity hard disk drives in recent years, various technologies have been developed. There are developed, for example, downsizing of a magnetic head or a magnetic head element, narrowing of tracks and sectors of the magnetic disk, thinning of various layers that are formed on the main surface of the magnetic disk, such as a protection/lubrication layer, a magnetic layer, and a soft magnetic layer, decreasing of a distance between the disk and the head, i.e., flying height, and increasing of a rotation speed of the magnetic disk.
However, when a glass substrate for a magnetic disk is produced by using the glass substrate having the pattern of the main surface satisfying the conventional definitions and a magnetic disk drive is produced by using the glass substrate, the flying height is reduced to about 8 nm and the rotation speed of the magnetic disk is set a speed as high as 10000 rpm or higher, an error such as a read error or a write error may occur in operation of the magnetic disk drive.